The production of micro-scale objects or components is growing in various industries. For example, biomedical devices such as catheters and biosensors are fabricated and packed with many miniature features to increase their capabilities to diagnose and treat diseases. In electronics industry, micro-electromechanical system (MEMS) devices are fabricated and incorporated with micro-scale features to enable the electronic, mechanical and electrical functionalities of those devices. In the aerospace industry, miniature components are produced and machined to precise dimensions and close tolerances to serve certain special purposes such as structural reinforcement and safety compliance. This growing need for the production of miniature components or micro-scale objects requires suitable metrology tools and systems, such as a coordinate measuring machine (CMM) or a probing system, to provide quality control mechanism for ensuring conformance of dimensions and surface profile with target specifications.
Generally, dimensional measurements and surface profiles of objects are determined by sensing or probing the surface using a probe through a non-contact or a contact method. The use of an optical probe or a laser probe provides a non-contact means to sense and scan the surface of an object. An example of laser-based method is shown in U.S. Pat. No. 4,7333,969, which describes a laser-based sensor system that can function as a CMM probe. While it provides a non-contact method, the optical or laser-based method is typically costly due to the use of expensive optics or laser system and the bulky structure required to house the whole system. Additionally, objects with intricate surface profile, such as those involving deep holes and complex curvatures, present a significant challenge to optical or laser-based method because those surface features can act as barriers that make the probe optically blind to some areas of interest.
On the other hand, the use of a tactile probe provides a contact method where measurements are determined when the probe establishes a physical contact with the surface of the object. An example of a tactile method and device is shown in U.S. Pat. No. 7,752,766. Typical design of a tactile probe involves a ball, such as a ruby or a steel sphere, as a probe tip that is attached to one end of the stylus. To improve the accuracy of measurements especially when conducting measurements in the micro-scale, the tactile probe needs to be fabricated to smaller sizes. Down-scaling the tactile probe usually leads to complex detection algorithms and a costly metrology tool. The finer the probe tip becomes, the higher is its fabrication cost. Also as the size of the tactile probe decreases, the probe becomes more susceptible to the “snap-in” effect. The “snap-in” effect occurs when the probe approaches within micrometers to the surface of the object, and is manifested by the sudden bending and adhesion of the probe to the surface of the object. This “snap-in” effect can introduce measurement errors and even damage the tactile probe. Likewise, frequent replacement of tactile probe from normal wear and tear or accidental damage can be costly.
Therefore, it is desirable to develop a micro-scale metrology tool, particularly a CMM or probing system, that is cost-effective and capable to provide precise measurements in the micro-scale. The CMM or probing system should be able to determine the precise spatial coordinates of multiple points along the contour of micro-scale solid objects and micro-scale features on bulk solid objects. By processing the recorded spatial coordinates using applicable mathematical algorithms, the dimensions, surface profile and construction of objects can be determined accurately.